There's a couple of things I like about them, and a few things I don't. There's some novel ideas, awesome ideas around living simply, living with less and so on. Storage seems to be a key issue.
Obviously they are very low cost - you are building something the size of a postage stamp, so your material costs are fairly low. That said it doesn't seem like many are built on standard sized woods etc (or at least not standard sizings in Australia, although I could be doing an imperial vs decimal misunderstanding).
They seem to be largely built out of wood. Is this because it's a home handyman DIY dream? Every one of these looks like a roving snack box to termites to me. Nom nom. And isn't the ongoing maintenance of wood a bit of a PITA - sealing, painting, water proofing, ant proofing, hard to seal tight etc?
If you are putting them on trailers and towing them around then they seem very bulky, chunky - all the wood - what about using sleeker materials, slim steel supports would get you as far with a lot less bulk - or is it again the aesthetic and the home handyman thing?
There are some great designs out there that are doable in a post peak oil world (or largely) - and can be built for under $300 USD. Generally they are using recycled housing materials, 'natural' wall components (clay, hay etc).
It seems most of the American ones rely heavily on modern technologies still - heaters/fires, airconditioners, chemical toilets and water heaters. Most are two story (to get the bed space in) - they are building up instead of out. If you are building in the wilds why make it that you have to climb a dinky ladder to crawl into a bed space that you can't sit up in? Why not add a wider floor plan and have a subsequently larger roof for more water collection. Which is another point - none of these (fixed ones) seem to have a water tank. Or even gutters. Or solar panels for roofing. Why not have solar panels for roofing, and the water runs down into a tank?
Finally - many of them are overly ornate. Complicated roof structures, dicky little porches, odd shaped windows. Not easy to construct (and partly why I'm confused these are in wood/for the home handyman), and turn from functionality to 'tweeness'.
I'm tempted to build one as a guest house/studio, and see how I like it - but the argument I'll have with my husband is "we've 5,000 acres, why build something so small and just have to come back and add onto it later?".